A classical mechanics problem by Pallav Doshi

Classical Mechanics Level 2

A body is rotating uniformly about a vertical axis fixed in an inertial frame. The resultant force on a particle of the body not on the axis is

horizontal and skew with the axis vertical horizontal and intersecting the axis none

1 solution

Vinayak Srivastava
Jul 19, 2020

@Aryan Sanghi , @Siddharth Chakravarty , how can this be calculated? I just guessed it and got it right. I wish to know the method. Thanks!

It is just centripetal force Centripetal force is always perpendicular to the axis of rotation and directed inwards, I.e. towards the axis. Check out the link for more details. :)

Aryan Sanghi - 10 months, 4 weeks ago

Thank you! But, I haven't started this chapter, it comes in Force and NLM, I suppose? I have not even started the questions of Kinematics! I think I'll understand better after reading it. Thanks for replying!

Vinayak Srivastava - 10 months, 4 weeks ago

Yes you should start from start. This question is of Rigid Body Dynamics, i.e. rolling motion. It'll come after NLM.

Aryan Sanghi - 10 months, 4 weeks ago

@Aryan Sanghi – Ok, do you have any special tip for solving Physics questions? I was studying Kinematics, and I could not solve any of the Solved Examples myself.

Vinayak Srivastava - 10 months, 4 weeks ago

@Vinayak Srivastava – See, many questions in physics are solved just using imagination. Imagine the situation. Rest questions, sketch the situation in the notebook and then start solving. Don't hesitate to see the solution of solved examples , you've just started. Hope it helps. :)

Aryan Sanghi - 10 months, 4 weeks ago

@Aryan Sanghi – Thank you! I'll try to remember it!

Vinayak Srivastava - 10 months, 4 weeks ago

@Aryan Sanghi has written it all @Vinayak Srivastava . You have to just understand the question and imagine. Here centripetal force(centre-seeking) force concept and rotational dynamics basically is used. Basically, the question asks what is the direction of the force of a particle, which is not on the axis i.e it is rotating. Now if you ever have taken something and tied to a string, you must have seen the end of string, or your tip of fingers are not moving, so the axis of rotation passes through there. But any other part, like the object tied is rotating so if you have ever seen when you leave the string from your hand, that object goes in a tangential direction, which means its velocity was tangential, but when it was rotating it did not go the reason is the centripetal force is towards your fingers which try to keep the object rotating. This is the reason why even moon doesnt fall to the earth or fly off, gravity is the centripetal force while the moon has a tangential velocity.

Siddharth Chakravarty - 10 months, 4 weeks ago

Thank you!

Vinayak Srivastava - 10 months, 4 weeks ago

A classical mechanics problem by Pallav Doshi

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